For a stock (or near stock) normally aspirated engine, especially an NPI (pre 99), the stock H-pipe is simply not that much of a restriction even peppered as it is with cats. Like fast97 says, the real gains in exhaust systems are long-tube headers and even then it's not so much because they are free-flowing but rather because their length and primary tube-size tunes the exhaust and promotes better cylinder scavenging.
Change to an X or H midpipe if you want to change the sound character of the car, not with an eye toward big HP gains. If you have emissions inspections in your area either go with a catted pipe or retain your factory H to re-install if you have emissions hassles. (I'd personally recommend holding on to the stock H-pipe no matter what, resisting the temptation to sell it on eBay or something...)